Reversal image development type electrophotographic printing system

Abstract

A reversal image development type electrophotographic printing system including a rotatable drum (1) coated with a light sensitive layer and an image developing roller (31) co-rotatable with the drum and using a printing medium composed of carriers and toner. To prevent superfluous toner during start-up, a predetermined bias voltage (-VB) differing to a normal bias voltage (+VB) is supplied to the image developing roller and the drum, and the image developing roller, a precharger (2), and power supplies (52, 53) for supplying the initial and normal bias voltages are energized in a predetermined start sequence. In addition, to prevent carrier extraction at a stop condition, the drum, the image developing roller, the precharger, and the power supply (53) are deenergized in a predetermined operation stop sequence. A printing medium including a resin toner comprising a mixed resin and magnetized powder and a charge control medium mixed with the resin toner may be used, and prevention of the surplus resin toner extraction in the start-up condition also may be achieved.

What is claimed is: 
     
       1. A two component reversal image development type electrophotographic printing system comprising: rotatable means having light sensitive means mounted thereon;   precharging means including:   a precharger provided adjacent to an outer surface of said rotatable means; and   a first power supply, connected to said rotatable means and said precharger, supplying a high voltage to said precharger and charging said light sensitive means to a predetermined voltage;   exposing means, connected to said light sensitive means, for exposing said charged light sensitive means to light beams for forming latent images in said charged light sensitive means;   developing means, connected to said light sensitive means, for developing images at portions of said light sensitive means on which said latent images are formed, said developing means including:   image developing roller means co-rotatable with said rotatable means and containing magnets and a sleeve covered thereon;   a second power supply, connected to said image developing roller means, for supplying a first bias voltage having a predetermined value which is equal to or greater than zero and having a polarity which is an inverted polarity of the high voltage from said first power supply to said image developing roller means for preventing a deposition of a printing medium on said light sensitive means until a portion of the light sensitive means precharged by the precharging means passes said image developing roller means; and   a third power supply, connected to said image developing roller means, for supplying a second bias voltage having a predetermined value and having the same polarity as the first power supply, to said image developing roller means for performing an image developing operation in a normal condition;   discharging means, connected to said light sensitive means, for discharging charges on said light sensitive means; and   controlling means, connected to said rotatable means and said image developing roller means, for controlling at least a start of said rotatable means and said image developing roller means, and energization and de-energization of said first, second and third power supplies, in a predetermined sequence in accordance with various positions of said precharging means, said image developing roller means, and said discharging means, and a rotational speed of said rotatable means, in at least the start-up condition.   
     
     
       2. A two component reversal image development type electrophotographic printing system according to claim 1, wherein said control means controls said rotatable means, said image developing roller means co-rotatable with said rotatable means, and said first, second, and third power supplies during the start-up condition by starting said rotatable means to rotate and energizing said second power supply, energizing said first power supply after the first time has elapsed, and energizing said third power supply and de-energizing said second power supply after the a second time has elapsed, the second time determined by when said portion of said light sensitive means facing said discharging means at the initial condition reaches said image developing means after said rotatable means starts rotating. 
     
     
       3. A two component reversal image development type electrophotographic printing system according to claim 2, wherein said control means includes timer means for counting said times. 
     
     
       4. A two-component reversal image development type electrophotographic printing system according to claim 3, wherein said rotatable means is a rotatable drum and wherein said light sensitive means is a light sensitive layer coated on said rotatable drum. 
     
     
       5. A two component reversal image development type electrophotograhic printing system according to claim 4, wherein said first time is determined when a portion of said light sensitive means facing said discharging means at an initial condition reaches said precharging means after said precharging means is energized. 
     
     
       6. A two component reversal image development type electrophotographic printing system according to claim 5, wherein said printing medium includes carriers and toner, said toner being electrostatically charged by friction electricity caused by agitating said carriers and said toner and having a same polarity as a charge on said light sensitive layer. 
     
     
       7. A two component reversal image development type electrophotographic printing system according to claim 6, wherein said control means further controls, in a stop condition, said rotatable means, said image developing roller means co-rotatable with said rotatable means and said first and third power supplies by de-energizing said first power supply, stopping said rotation of said rotatable means, and de-energizing said third power supply, after a third time has elapsed, determined when a portion of said light sensitive layer facing said discharging means at the beginning of a stop time reaches said image developing roller means. 
     
     
       8. A two component reversal image development type electrophotographic printing system according to claim 7, wherein said control means includes a timer means for counting said third time. 
     
     
       9. A two component reversal image development type electrophotographic printing system according to claim 4, wherein said first time is zero. 
     
     
       10. A two component reversal image development type electrophotographic printing system according to claim 9, wherein said printing medium includes carriers and toner, said toner being electrostatically charged by friction electricity caused by agitating said carriers and said toner and having a same polarity as a charge on said light sensitive layer. 
     
     
       11. A two component reversal image development type electrophotographic printing system according to claim 10, wherein said control means further controls, in a stop condition, said rotatable means, said image developing roller means co-rotatable with said rotatable means and said first and third power supplies by de-energizing said first power supply, stopping said rotation of said rotatable means and de-energizing said third power supply, after the elapse of a third time determined when a portion of said light sensitive layer facing said discharging means at the beginning of a stop time reaches said image developing roller means. 
     
     
       12. A two component reversal image development type electrophotographic printing system according to claim 11, wherein said control means includes a timer means for counting said third time. 
     
     
       13. A two component reversal image development type electrophotographic printing system according to claim 4, further comprising a printing medium, wherein said printing medium includes resin toner comprising a mixed resin and magnetized powder and a charge control medium mixed with said resin toner, said resin toner being electrostatically charged by said sleeve on said developing roller means. 
     
     
       14. A two component reversal image development type electrophotographic printing system according to claim 13, wherein said first time is determined when a portion of said light sensitive means facing said discharging means at an initial condition reaches said precharging means after said precharging means is energized. 
     
     
       15. A two component reversal image development type electrophotographic printing system according to claim 13, wherein said first time is zero. 
     
     
       16. A two component reversal image development type electrophotographic printing system according to claim 1, wherein said high voltage of said first power supply is approximately 5 KV to 6 KV, and charges of approximately 500 V to 600 V are supplied to said light sensitive means, which are reduced to approximately 0 V to 100 V by said exposure to said light beams, said first bias voltage of said second power supply being -50 V to -100 V and said second bias voltage of said third power supply being approximately 300 V. 
     
     
       17. A two component reversal image development type electrophotographic printing system according to claim 16, further comprising: paper;   a cleaner in contact with said light sensitive means;   an image transfer device, connected to said light sensitive means, for transferring toner of said printing medium deposited on said light sensitive means to said paper;   a paper separator, located between said paper and said light sensitive means, for separating said paper attached to said light sensitive means; and   an image fixing device, connected to said paper, for fixing said toner on said paper.